There has been an increasing need for expanding the capacity of an optical transmission path. As one of techniques for realizing the capacity expansion of an optical transmission path, WDM (Wavelength Division Multiplexing) has been put into practice. In the WDM system, a plurality of data signals are transmitted through an optical fiber, using a plurality of different wavelengths. However, the speed of an optical signal propagated through the optical transmission path is dependent on the wavelength. For this reason, when, for example, a plurality of optical signals transmitted from a plurality of clients are multiplexed by a WDM apparatus and transmitted to a server, a long distance between the WDM apparatus and the server would result in the optical signals arriving at the server one another at different timing. In this case, with some applications, the server has to wait for the arrival of all optical signals before it can start data processing, which would hinder high-speed processing. Therefore, if an application is to be affected by a transmission delay-time difference in the optical transmission path, the signal capacity needs to be expanded, not in the WDM transmission, but in TDM (Time Division Multiplexing) transmission.
In order to expand the signal capacity in the TDM transmission, the pulse width of the optical signal needs to be narrow. In other words, the data needs to be transmitted using RZ (Return to Zero) modulation. For the current optical communication systems, however, the data modulation is performed mainly using NRZ (Non-Return to Zero) method. Therefore, a technique for converting an NRZ signal to an RZ signal plays an important role. Meanwhile, a pulse light source for generating a very short pulse for realizing high-speed communication is generally expensive and the apparatus is large in size.
FIG. 1 is a diagram illustrating an example of an optical communication system in which an NRZ signal is converted into an RZ signal and then transmitted in TDM. In FIG. 1, each conversion circuit 1 converts an NRZ signal output from a corresponding transmission apparatus into an RZ optical signal. A TDM apparatus 2 multiplexes the plurality of RZ optical signals and transmits the multiplexed signal to a receiving apparatus. Recently, a system has been reported, in which a several-dozen-Gbps TDM-RZ optical signal is generated and transmitted, by multiplexing several-Gbps NRZ optical signals.
FIG. 2 is a diagram illustrating an example of a conventional optical NRZ/RZ conversion circuit. In FIG. 2, an O/E conversion element 11 converts an NRZ optical signal into an electric signal. Here, the bit rate of the NRZ optical signal is assumed to be B [bps]. A retiming circuit 12 recovers a clock signal from the electric signal obtained by the O/E conversion element 11. The frequency of the recovered clock signal is B [Hz]. At this time, jitter in the clock signal can be removed by the retiming circuit 12. The clock signal is provided to an intensity modulator 14 through a delay element 13. The intensity modulator 14 performs intensity modulation for the NRZ optical signal using the provided clock signal, and converts the NRZ optical signal into an RZ optical signal. The configuration is capable of converting an NRZ signal into an RZ signal while removing its jitter. An optical NRZ/RZ conversion circuit with the interposition of an electric signal is described in, for example, Patent Document 1 (Japanese Examined Patent Application Publication No. 7-95756) and Patent Document 2 (Japanese Patent Application Publication No. 2005-252805).
FIG. 3 is a diagram illustrating another example of a conventional optical NRZ/RZ conversion circuit. In FIG. 3, an NRZ optical signal is amplified by an optical amplifier 21 and then enters a nonlinear optical medium 22. The nonlinear optical medium 22 is an optical fiber such as a dispersion decreasing fiber. The pulse width of an optical signal is compressed in the nonlinear optical medium 22 by the negative dispersion and an adiabatic compression effect generated through the interaction due to the nonlinear effect. In other words, in the nonlinear optical medium 22, when the length of the nonlinear optical medium 22 and the nonlinear coefficient are determined appropriately, an NRZ optical signal is converted into an RZ optical signal. Since this configuration does not involve the interposition of an electric signal and the response time of the nonlinear effect of the optical fiber is very short, the pulse width of the optical signal can be compressed to less than one picosecond.
Meanwhile, for the optical communication system using TDM, a transmission rate equal to or more than 100 Gbps is expected to be required in the future. In that case, an optical signal having a pulse width equal to or less than one picosecond will be required. In addition, jitter in the optical signal needs to be removed, or suppressed sufficiently.
However, since the optical NRZ/RZ conversion circuit illustrated in FIG. 2 is configured to compress the optical pulse through a gate operation using an electric signal (i.e., the intensity modulation), the pulse width cannot be compressed sufficiently, due to the influence of the speed limitation in the electric circuit. The pulse width that can be obtained in a conventional electric circuit is limited to about 10 picoseconds. Meanwhile, although the pulse width can be compressed sufficiently in the NRZ/RZ conversion circuit illustrated in FIG. 3, jitter cannot be removed.
Thus, according to the conventional art, it has been difficult to generate a very short pulse (RZ optical signal) for high-speed optical communication, from an NRZ optical signal.
Meanwhile, Patent Document 3 (Japanese Patent Application Publication No. 2005-241902) describes, while it is not a technique for converting an NRZ optical signal into an RZ optical signal, a technique for generating an optical pulse for high-speed optical communication.